1. Field of the Invention
The present invention relates generally to print media feeding for hard copy printing and plotting apparatus and, more particularly, to a cut-sheet, print medium, pick and feed mechanisms.
2. Description of Related Art
Many office products such as computer printers and plotters, plain paper facsimile machines, and photocopiers use mechanisms that pick a sheet of paper from a stack of media in an input tray and feed a single sheet of pre-cut printing medium (for example, a sheet of paper of a particular size such as standard letter size, legal size, or A4 (metric), or transparencies, or envelopes) into the hard copy producing apparatus. These mechanisms are generically referred to as "sheet feeders."
Sheet feeders usually are provided with an adjustable or replaceable media cartridge, tray, or other type of stacker in which a user can stack multiple cut-sheets of the media of choice. The use of media cartridges (essentially easily substituted paper trays) adapted to the various styles of media provide a mechanism for quick changes between any particular printing medium by the user. Upon receiving a media feed command ("FEED") from the hard copy machine controller electronics, a sheet picking device is actuated to deliver the top sheet from the stack into the hard copy machine. Under proper operating conditions, a sheet picking mechanism associated with the stacker should deliver a single sheet of the print medium to the hard copy machine input mechanism, such as a set of pinch rollers used for registration and feed into the actual printing subsystem of the machine.
Misfeeds, multiple sheet feeds, paper jams, and the like are common problems associated with sheet feeders. FIGS. 1 and 1A depict a typical problem of a misfeed caused by a premature bending ("wrap" or "wrap-back") of a picked sheet before its leading edge can be captured by an input mechanism (not shown; generally located close to the exit plane of the leading edge of a picked sheet) used to transport the sheet to a printing station. FIG. 1 shows a normal feed of a sheet 100 from a stack 101 of sheets in an input bin 102 by a pick mechanism 103 having a feed arm 104 and a motorized pick roller 105. In FIG. 1 the sheet 100 is being properly fed up the input tray angled wall 106. The details of a stack loader located below a hard copy apparatus' printing mechanisms are shown in U.S. Pat. No. 5,507,478 by Nottingham et al. for PRINTING MEDIA STATUS SENSING, assigned to the common assignee herein and incorporated herein by reference.
FIG. 1A shows a misfeed in which the picked sheet 100 has wrapped around the pick mechanism 103 before properly exiting the bin and being received by the input feed mechanism. High humidity tends to exacerbate such a problem. Thin media or media that has any inherent curl in it is highly susceptible to the problem.
One solution to the problem is to flatten out the feed angle. U. S. Pat. No. 5,876,133 by Kleln et al. for a SHEET PRESENTER AND METHOD OF USING SAME has an elongated platform adapted to engage a printer. A pair of spaced walls extend upwardly from the platform to support pivotally an arm at one end thereof. A motor disposed at another end of the arm is operatively connected to a drive roller wherein the mass of the motor urges the corresponding end of the arm downwardly to engage a sheet. This however requires a much larger workspace footprint than a printer having a contained input tray located below the printing zone such as illustrated by FIGS. 1 and 1A and preferable in the marketplace.
Another solution is to provide a paper guide fixed to a pivotal feed arm 104. However, such paper guides move with the arm. When the rollers of the feed arm are on a full stack, the paper guides protrude above the highest level of the feed arm, requiring extra height in the media bin to accommodate them. If the guides are made to be very short to minimize the amount that they protrude above the feed arm's highest position, they will not be effective when the paper stack is low and the feed arm pivoted into a more vertical than horizontal orientation. The picked sheet of a low stack will tend even more to wrap around the feed arm before reaching the top of the wall 106 where it can be captured by transport rollers.
Therefore, there is a need to facilitate the transfer of a sheet of print media from an internal input supply stack to a hard copy machine's printing zone.